NASA's Advanced Stitching Machine is located at the Marvin B.
Dow Stitched Composites
Development Center--a new Boeing facility that will produce
low-cost composite wing structures.

From the World War II image of "Rosie the Riveter" bolting
together war planes to the jets flying overhead today, Americans
have always associated metal airplanes with strength. However,
aircraft designers today are turning to composite materials to meet
the growing challenge of maintaining safety and economy for
commercial air travelers. Anticipating this challenge, NASA and
Boeing have joined forces under the NASA Advanced Composites
Technology (ACT) program to make large composite airplane
structures a reality.

Here, the full span of the stitched/RFI wings is shown.
Composites will make up all but the leading and trailing
edges of the wing. The largest portion of the wing, called
the wing stub-box (shown bottom right), was tested at
NASA Langley in July, 1995.

The ACT Program

The NASA ACT program was set up in 1989 to improve the
efficiency of composite structures and to reduce their
manufacturing costs. The program aims to reduce air travel costs
through the use of composite materials on commercial aircraft.

The Advanced Stitching Machine (ASM) was designed and built
under the ACT program to aid in making large structures out of
composites. The goals of the ACT initiative are to make composite
wing structures 25 percent lighter, to reduce production costs by
20 percent and to reduce operating costs to airlines. The ACT
program will develop the scientific basis required for FAA
certification of composite wings. The program will also help
accomplish one of NASA's new technology goals for aeronautics--to
reduce the cost of air travel by 25 percent within 10 years, and by
50 percent within 20 years.

Background

NASA's early composites research provided the aircraft builders
with important technology but the industry lacked the confidence to
use laminated composites to manufacture wing and fuselage
structures. The barrier issues were high cost and low damage
tolerance. Industry wanted composite structures that cost less than
aluminum and that were robust enough to withstand the rigors of
airline services. However, low damage tolerance remained an issue
despite major efforts to develop new tough epoxy resins.

In the 1980s researchers looked to textile composites as
breakthrough technology. Supporters argued for new concepts which
would use knitting, weaving, braiding and through-the-thickness
stitching for reinforcement and use existing U.S. textiles
manufacturing technology for cost efficiency.

Under the ACT program, various types of textile composites were
thoroughly tested but it was stitching combined with resin film
infusion that showed the greatest potential for overcoming the cost
and damage tolerance barriers to wing structures. Assembling carbon
fabric preforms, (pre-cut pieces of material), with closely spaced
through-the-thickness stitching provided essential reinforcement
for damage tolerance. Also, stitching made it possible to
incorporate the various elements--wing skin, stiffeners, ribs and
spars--into an integral structure that would eliminate thousands of
mechanical fasteners. Although studies showed that stitching had
the potential for cost-effective manufacturing, the critical need
was for machines capable of stitching large wing preforms at higher
speeds.

Evolution of Textile Technology

A primitive single-needle stitching machine, resembling a
scaled-up version of a household sewing machine, was the first
prototype used to determine the benefits of stitched composites.
This initial research identified that stitched composites offered
better levels of damage tolerance than conventional laminated
composites.

This single-needle sewing machine was used in exploratory
research on stitched composites.

Under a six-year NASA ACT contract, Boeing chose the stitching
of dry textile fabrics, in conjunction with the resin film infusion
(RFI) process to develop cost-effective wing structures. For
stitching the skins of large test panels, a multi-needle quilting
machine was obtained and modified to demonstrate a manufacturing
approach to stitching layers of carbon fabric. Although the
multi-needle machine served important needs in the wing
development, it was relatively slow and unable to stitch thick
layers of fabric.

The next step in the development was a computer-controlled
single-needle gantry machine that could stitch through the thick
carbon fabrics. Both the multi-needle and single-needle gantry
stitching machines had unique features and capabilities; however,
neither were designed with the capability to quickly stitch large,
complex contoured wing structures.

NASA awarded Boeing a contract to develop a larger machine
capable of stitching entire wing covers for commercial transport
aircraft. This high-speed, multi-needle machine, known as the
Advanced Stitching Machine (ASM), was designed and built under the
NASA ACT wing program. Under subcontract to Boeing, Ingersoll
Milling Machine Company, Rockford, IL, was selected to design and
build the ASM. The ASM's advanced stitching heads were designed and
built by Pathe Technologies, Inc., Irvington, NJ.

Concurrent with the development of the large stitching machine,
NASA and Boeing proceeded with a building block approach to
demonstrate the design and manufacture of stitched/RFI wing
structures.

The largest of the demonstration sections was a 12-ft. long wing
stub box which was fabricated at Long Beach, CA, and tested at NASA
Langley Research Center in July 1995. The wing stub box manufacture
demonstrated that the stitching/RFI concept could be used to make
the thick composite structures needed for heavily loaded wings. The
successful test of the stub box proved the structure and damage
tolerance of a stitched wing.

New Composites Manufacturing Process

Knitting

The basic "skin" of the composite wing upper and lower covers is
made using knitted carbon-tow fabrics. A commercial supplier
delivers multiaxial warp knit fabrics stacked as specified by
Boeing. Next, the stacks are cut into the pieces that form the
shape of the wing (preforms). The knitted preforms are stacked with
as few as two stacks in low-stress areas and up to twenty stacks in
high-stress areas. When the fabric pieces are arranged in the
proper position, the ASM stitches the stacks to make a solid wing
preform.

Braiding

The stiffeners and rib clips for wing covers are made using a
braiding process which makes it easier for them to conform to the
contours of the wing. Braided tubes are collapsed and stitched to
make blade-shape stiffeners and rib clips.

Stitching

In a final step, the ASM stitches the stiffening elements to the
skin preform. The result is an integral wing cover preform, shaped
to the wing contours, ready for the RFI process.

The Role of the ASM

The ASM is an integral part of the stitched/RFI composites
manufacturing process. With through-the-thickness technology, the
ASM's stitching heads can penetrate through textile preforms 1.5"
thick. This type of stitching increases damage tolerance and load
capability especially when assembling and binding secondary
materials--stiffeners, spar caps and intercostal clips--onto the
skin.

Stitching materials together is also faster than drilling holes
and assembling the 80,000 metal fasteners found on an aluminum
wing. Removal of this excess metal decreases the weight of the wing
and eliminates the problems of fatigue or corrosion of the metal
fasteners.

The RFI Process

When the stitching is complete, the still flexible wing skin
panel is put into an outer mold line (OML) tool that is the shape
of the outside surface of the wing. A film of resin is laid on the
OML form, followed by the composite skin panel and the tools that
will define the inner mold line.

These elements are put into a plastic bag from which the air is
drawn out, creating a vacuum.

The materials are then placed in an autoclave, where heat and
pressure are applied to let the resin spread throughout the carbon
fiber material. After heating to 350 degrees for two hours, the
wing skin panel takes on its final hardened shape.

The stitched/RFI method differs from conventional composite
methods in which the composite material with the resin already
impregnated (pre-preg) is laid down on a tool before being put into
the autoclave. The new RFI process eliminates the cost of
conventional pre-pregging and its time-critical setup.

Analytical models are used to predict several factors in the RFI
process to reduce the risk of failure. Engineers have to factor in
the viscosity of resin, compaction and permeability of the fabric
materials, and length of processing time before the resin sets.

The Future of The ACT Program

The panels currently being stitched on the ASM will eventually
be used as test articles in a full-scale ground test of a composite
wing for an airliner. A test of this forty-foot semi-span wing will
take place at NASA Langley's Structures and Materials Laboratory in
1999. The tests will simulate various levels of damage to ensure
that the composite wing meets FAA standards. The ACT program will
be completed in the year 2001.

Advanced Stitching Machine

In this cost-sharing effort, NASA has spent $10 million on the
development of the ASM and Boeing has paid for the renovations at
the Marvin B. Dow Stitched Composites Center at Huntington Beach,
CA, where the ASM will be housed. (The building had to be modified
for the huge machinery of the ASM, with the inclusion of
specialized equipment.)

The ASM features high speed stitching capability with advanced
automation allowing it to stitch large, thick, complexwing
structures without manual intervention.

Equipped with four stitching heads, this massive machine is able
to stitch one-piece aircraft wing cover panels 40-feet long, 8-feet
wide and 1.5-inches thick at a rate of 3,200 stitches per minute.
The stitching heads also offer machine tool precision, stitching at
8 stitches per inch with a row spacing of .2 inches.

The ASM's 50 lift tables can be seen outstretched
behind the stitching heads in front.

However, to achieve this rate, a pivoting or walking needle
mechanism and needle cooling system had to be developed. These
improvements prevented excessive needle bending and associated
temperature build-up in the needle. In addition, to maintain
desired stitching speeds, an automated thread gripper and cutting
mechanism was developed.

A technological marvel, the ASM has computers controlling 38
axes of motion. The computers are also used to simulate and confirm
the stitching pattern on the 50-foot bed of the ASM.

A laser projection system is used to precisely locate the wing
skin on the lift table surface before stitching begins. This same
aerospace precision is used to locate secondary materials, like the
stiffeners, for stitching.

The movements of the stitching heads are synchronized with each
of the fifty lift tables it takes to control stitching over the
contoured shapes of the wing panels. The lift tables are used to
support the dry fabric preforms as they are stitched.

The ASM is capable of stitching wing cover panels in one,
two-shift operation saving days over conventional composite
manufacturing processes. Cost analyses indicate that a reduction of
20 percent in cost can be achieved over equivalent wings built from
aluminum.

This, together with the reduction in weight, translates to a
much improved competitive position for airlines in the global
market and ultimately a reduction in future air travel costs.

Marvin B. Dow Stitched Composite Development Center

Boeing named its new Stitched Composite Development Center after
NASA Langley researcher Marvin B. Dow in honor of his contributions
to stitched composites research and, specifically, to the Advanced
Stitching Machine (ASM). Dow spent the last 25 years of his 40-year
NACA/NASA career in pursuit of the application of advanced
composite materials on commercial transport aircraft. He is the
first NASA employee honored in the naming of a corporate
facility.

Dow's composites research began in the 1970s. His work on
composites led to the flight testing of graphite/epoxy rudders on
the McDonnell Douglas DC-10 commercial transport aircraft.

During the next 10 years as a key technical leader in the NASA
AirCraft Energy Efficiency (ACEE ) Program, he was instrumental in
developing composites technology for application to structures on
DC-10, B-737 and C-130 aircraft.

In the late 1980s, Dow conducted pioneering research on
innovative reinforcement concepts that would lead to improved
damage tolerance and reduced cost compared to state-of-the-art
composite manufacturing methods. This research focused on textile
reinforcement concepts such as weaving, braiding, knitting,
stitching and resin transfer processes. In 1989, Dow became the
technical manager of the NASA ACT Wing Program. Dow worked with
Boeing, (then McDonnell Douglas), to develop the ASM and the
stitched/resin film infusion process. His vision of large-scale
automated stitching technology finally came to fruition with the
success of the ASM. The ASM--made possible by Dow's long-term
dedication--is expected to revolutionize the way aircraft wing
structures are fabricated.

Marvin Dow, Distinguished Research Associate with
NASA Langley, spearheaded the development of the ASM.

Dow retired from NASA Langley in September 1996 and currently
serves the Center as a Distinguished Research Associate. His latest
project is a technical summary and bibliography titled, "The
Development of Stitched, Braided and Woven Composite Aircraft
Structures in the U.S. (1985 to 1997)."